Facial Action Coding System (FACS) – A Visual Guidebook

In this article, we have put together a visual guidebook to better showcase the power of the facial action coding system (FACS). All facial action units are presented with animations to give a first-hand understanding and valuable reference point in the future. So if you are working with facial expression analysis (FEA) we encourage you to bookmark this page – it could just save you a lot of work.

The Facial Action Coding System

The Facial Action Coding System (FACS) refers to a set of facial muscle movements that correspond to a displayed emotion. Originally created by Carl-Herman Hjortsjö with 23 facial motion units in 1970, it was subsequently developed further by Paul Ekman, and Wallace Friesen. The FACS as we know it today was first published in 1978, but was substantially updated in 2002.

Using FACS, we are able to determine the displayed emotion of a participant. This analysis of facial expressions is one of very few techniques available for assessing emotions in real-time (fEMG is another option). Other measures, such as interviews and psychometric tests, must be completed after a stimulus has been presented. This delay ultimately adds another barrier to measuring how a participant truly feels in direct response to a stimulus.

Researchers have for a long time been limited to manually coding video recordings of participants according to the action units described by the FACS. This process is now possible to complete with automatic facial expression analysis. This saves vast amounts of time and money, as scoring no longer requires analysis of each frame by a trained researcher – the software simply does the work for you.

Below we have listed the major action units that are used to determine emotions. Roll your mouse over the image to start the movement!

Main Action Units

Action Unit	Description	Facial Muscle	Example
1	Inner Brow Raiser	Frontalis, pars medialis
2	Outer Brow Raiser (unilateral, right side)	Frontalis, pars lateralis
4	Brow Lowerer	Depressor Glabellae, Depressor Supercilli, Currugator
5	Upper Lid Raiser	Levator palpebrae superioris
6	Cheek Raiser	Orbicularis oculi, pars orbitalis
7	Lid Tightener	Orbicularis oculi, pars palpebralis
9 (also shows slight AU4 and AU10)	Nose Wrinkler	Levator labii superioris alaquae nasi
10 (also shows slight AU25)	Upper Lip Raiser	Levator Labii Superioris, Caput infraorbitalis
11	Nasolabial Deepener	Zygomatic Minor
12	Lip Corner Puller	Zygomatic Major
13	Cheek Puffer	Levator anguli oris (Caninus)
14	Dimpler	Buccinator
15	Lip Corner Depressor	Depressor anguli oris (Triangularis)
16 (with AU25)	Lower Lip Depressor	Depressor labii inferioris
17	Chin Raiser	Mentalis
18 (with slight AU22 and AU25)	Lip Puckerer	Incisivii labii superioris and Incisivii labii inferioris
20	Lip stretcher	Risorius
22 (with AU25)	Lip Funneler	Orbicularis oris
23	Lip Tightener	Orbicularis oris
24	Lip Pressor	Orbicularis oris
25	Lips part	Depressor Labii, Relaxation of Mentalis (AU17), Orbicularis Oris
26 (with AU25)	Jaw Drop	Masetter; Temporal and Internal Pterygoid relaxed
27	Mouth Stretch	Pterygoids, Digastric
28 (with AU26)	Lip Suck	Orbicularis oris
41	Lid droop	Relaxation of Levator Palpebrae Superioris
42	Slit	Orbicularis oculi
43	Eyes Closed	Relaxation of Levator Palpebrae Superioris
44	Squint	Orbicularis oculi, pars palpebralis
45	Blink	Relaxation of Levator Palpebrae and Contraction of Orbicularis Oculi, Pars Palpebralis.
46	Wink	Levator palpebrae superioris; Orbicularis oculi, pars palpebralis

Head Movement Action Units

Action Unit	Description	Example
51	Head Turn Left
52	Head Turn Right
53	Head Up
54	Head Down
55	Head Tilt Left
56	Head Tilt Right
57	Head Forward
58	Head Back

Eye Movement Action Units

Action Unit	Description	Example
61	Eyes Turn Left
62	Eyes Turn Right
63	Eyes Up
64	Eyes Down

Emotions and Action Units

The Action Units described above show the different movements of facial muscles. Certain combined movements of these facial muscles pertain to a displayed emotion. Emotion recognition is completed in iMotions using Affectiva, which uses the collection of certain action units to provide information about which emotion is being displayed. For example, happiness is calculated from the combination of action unit 6 (cheek raiser) and 12 (lip corner puller). A complete list of these combinations and the emotion that they relate to is shown below. The gifs on the right are shown in the same order that the action units listed.

Emotion	Action Units	Description	Examples
Happiness / Joy	6 + 12	Cheek Raiser, Lip Corner Puller
Sadness	1 + 4 + 15	Inner Brow Raiser, Brow Lowerer, Lip Corner Depressor
Surprise	1 + 2 + 5 + 26	Inner Brow Raiser, Outer Brow Raiser, Upper Lid Raiser, Jaw Drop
Fear	1 + 2 + 4 + 5 + 7 + 20 + 26	Inner Brow Raiser, Outer Brow Raiser, Brow Lowerer, Upper Lid Raiser, Lid Tightener, Lip Stretcher, Jaw Drop
Anger	4 + 5 + 7 + 23	Brow Lowerer, Upper Lid Raiser, Lid Tightener, Lip Tightener
Disgust	9 + 15 + 16	Nose Wrinkler, Lip Corner Depressor, Lower Lip Depressor
Contempt	12 + 14 (on one side of the face)	Lip Corner Puller, Dimpler

Putting it all together

When measuring facial expressions within iMotions, the stimuli are paired automatically to the FACS analysis, allowing you to pinpoint the exact moment that the stimulus triggered a certain emotion. The FACS is also graded on a scale of intensity, which gives a measure of how strongly the emotion is displayed. These measurements can also be synchronized with recordings of galvanic skin response, which provides a measure of arousal. With this information combined, it’s possible to start drawing conclusions about how strongly an individual felt, and what those emotions consisted of, in response to a set stimulus.

The screenshot below shows how the facial expression data is displayed while a participant watches an advertisement.

If we zoom in, we can see the intensity of the displayed emotion. There are five emotions displayed in the image below, however iMotions provides a measure of the seven central emotions (shown in the table above), alongside, and in conjunction with measurements of action units.

I hope this explanation of action units and FACS has been helpful, and informative. If you’d like to learn even more about facial expressions, then we also have a free pocket guide that you can download for free below!